1. Field of the Invention
The present invention relates to a method for manufacturing a wafer including a superficial silicon layer, an insulating layer containing oxide, and a silicon support arranged in that order. This type of wafer is referred to as a silicon-on-insulator (SOI) wafer.
2. Description of the Related Art
In general, a separation-by-implanted oxygen (SIMOX) process or a bonding process is used to manufacture SOI wafers including superficial silicon layers, insulating layers containing oxide, and silicon supports arranged in that order. In the SIMOX process, oxygen ions are implanted into a silicon wafer and the resulting silicon wafer is heat-treated at a predetermined temperature, whereby an insulating layer is formed in the silicon wafer. Such an insulating layer is referred to as a buried oxide (BOX) layer. In the bonding process, an oxide layer is formed on a first silicon wafer to be processed into a superficial silicon layer and then bonded to a second silicon wafer to be processed into a silicon support and the first silicon wafer is reduced in thickness such that the first silicon wafer has a predetermined thickness.
In particular, for the SIMOX process, the silicon wafer containing the oxygen ions is placed in a furnace and then heat-treated at, for example, 1,350° C. or more in a heat-treating step, whereby the insulating layer is formed. In the heat-treating step, oxygen or a gas mixture containing argon and a small amount of oxygen is supplied to the furnace. Therefore, the silicon wafer is heat-treated in an atmosphere containing oxygen or argon together with a small amount of oxygen. The heat-treated silicon wafer is taken out of the furnace in an unloading step. In the unloading step, gas in the furnace is replaced with nitrogen and the temperature of the furnace is reduced to 800° C. or less. Since large-diameter wafers having a diameter of, for example, 300 mm have been recently used, the temperature of the furnace is usually reduced to 600° C. or less in the unloading step. This is because slip that causes a deterioration in the quality and/or yield of SOI wafers must be prevented from occurring.
On the other hand, for the bonding process, the first and second silicon wafers and the oxide layer are placed in a furnace and then heat-treated at, for example, 1,000° C. or more in a heat-treating step, whereby the second silicon wafer and the oxide layer are securely bonded to each other; that is, the interface between the second silicon wafer and the oxide layer is enhanced in bonding strength. This leads to the formation of an SOI wafer. In the heat-treating step, nitrogen or oxygen is supplied to the furnace and the first and second silicon wafers and the oxide layer are therefore heat-treated in a nitrogen or oxygen atmosphere. The SOI wafer is taken out of the furnace in an unloading step. In the unloading step, gas in the furnace is replaced with nitrogen as described about the SIMOX process and the temperature of the furnace is reduced to 800° C. or less. As described about the SIMOX process, since large-diameter wafers having a diameter of, for example, 300 mm have been recently used, the temperature of the furnace is usually reduced to 600° C. or less in the unloading step.
In the bonding process, after the unloading step is performed, in order to remove an oxide film present on the first silicon wafer, in order to remove defects from the first silicon wafer, and in order to adjust the thickness of the first silicon wafer, the first silicon wafer is polished in a polishing step, whereby the superficial silicon layer is obtained.
After the heat-treating step is performed, in order to adjust the thickness of the SOI wafer, a sacrificial oxide layer may be formed on the superficial silicon layer, whereby the thickness of the superficial silicon layer is reduced. This step may be also performed when the SIMOX process is used. In the sacrificial oxide layer-forming step, the SOI wafer is placed in a furnace to which oxygen is supplied, whereby the SOI wafer is heat-treated at, for example, 1,000° C. or more in an oxygen atmosphere. After this step is performed, a subsequent unloading step as well as the unloading step performed subsequent to the heat-treating step is performed.
The inventors have investigated properties of the interfaces between superficial silicon layers and insulating layers of SOI wafers prepared by a known method. The investigation showed that the electron mobility of the interfaces therebetween was low because the interfaces had high interface state density. That is, the electron mobility of the interfaces is in inverse proportion to the interface state density.
If electronic devices such as transistors are manufactured from SOI wafers having high interface state density, such electronic devices suffer from the following problems: a shift in threshold voltage, an increase in subthreshold characteristic, and a deterioration in performance. That is, the electronic devices are unsatisfactory in performance and reliability.
In order to manufacture electronic devices having high performance and reliability using SOI wafers, such SOI wafers must have low interface state density.
Characteristic electronic defects at the Si—SiO2 interface, Appl. Phys. Lett., 1983, 43, p. 563 (hereinafter referred to as Non-patent Document 1) discloses that the increase in the interface state density at the Si/SiO2 interfaces is caused by free bonds of silicon atoms present at the interfaces, the free bonds being usually referred to as dangling bonds. Passivation and depassivation of silicon dangling bonds at the Si/SiO2 interface by atomic hydrogen, Appl. Phys. Lett., 1993, 63, p. 1510 (hereinafter referred to as Non-patent Document 2) discloses a technique for terminating dangling bonds with atomic hydrogen. This technique is useful in reducing the number of dangling bonds at the Si/SiO2 interface. Therefore, the following procedure is probably effective in reducing the interface state density at the Si/SiO2 interface: an SOI wafer is prepared and then heat-treated in a hydrogen atmosphere, whereby hydrogen is introduced into the interface between a superficial silicon layer and insulating layer of the SOI wafer.
Japanese Unexamined Patent Application Publication No. 2002-26299 discloses the following procedure: nitrogen precipitates containing a predetermined amount of nitrogen are provided on an SOI wafer for fabricating electronic devices, whereby the interface state density at the Si/SiO2 interface is reduced.
For the technique and procedures described above, the number of manufacturing steps is large; hence, a manufacturing system is complicated and manufacturing cost is high. Therefore, the following method is demanded: a simple method for manufacturing an SOI wafer having low interface state density.